Aircraft engines (both reciprocating piston and turbine) which drive a propeller, or ducted fan, are typically installed in a fuselage and/or wings. The disposition of the engine installation may be forward or rearward, in relation to the aircraft longitudinal axis and its leading or trailing aerodynamic (lift) surfaces. Similarly, the engine orientation and attendant propeller may be puller or pusher configurations.
An engine represents a concentrated mass of robust form compared with an airframe structure and so an engine mounting must “marry” the engine and airframe, by distributing imposed loads.
In both fuselage and wing installation, an engine is typically mounted upon a bulkhead, disposed at the opposite end from the thrust propeller, or upon a structure extended beneath and/or alongside the engine. In the case of a fuselage, the bulkhead separates the engine and passenger or baggage compartments.
A requirement for low noise and/or vibration transmission from the engine to occupants in the passenger compartment commonly dictates the use of compliant, for example, metallic or elastomeric, mountings in supporting an engine assembly from or upon an airframe. Essentially the mount configurations depicted in FIGS. 1A through 3B and FIGS. 12 and 13 of the patent drawings disclose known aircraft engine mount systems. These mounting systems include a “bed mount” system which is reliant upon engine support from below by an extended underlying structure (FIGS. 1A and 1B); a side mount system which is used with in-line engines and involves two mounts to either side of the engine and a beam or truss structure extending from the aircraft bulkhead (FIG. 12); and an end mount system with an engine cantilevered from one end opposite a thrust (pusher or puller) propeller (FIGS. 2A-2B, 3A-3B, and 13A-13B). Both the “bed” and “side” mount systems include considerable supplementary structure beyond (forward of) a bulkhead.
For low cost and simplicity, an end arrangement is preferable since it minimizes airframe structural extension. Thus, with an end-mount, a minimal intervening frame between engine and bulkhead can be employed.
An end mount with multiple individual mount axes disposed around and each orientated substantially parallel to the engine thrust axis, i.e. a so-called “axial mount” represents a simple solution. In this case, each individual (axial) mount is loaded (transversely) in shear under aircraft normal maneuvering load (FIGS. 2A-2B).
A primary vibration transmission mode arises from the pulsing torque reaction of the engine, particularly with a piston engine where the power generation is cyclical or intermittent. Individual axial mountings of an end mount configuration are shear loaded by oscillatory torque reaction while having greater tension or compression stiffness in order to withstand propeller thrust reaction and maintain alignment of the propeller with the aircraft (longitudinal) axis. A low mount shear stiffness while advantageous for absorption, cushioning or reduced transmission of (torque) vibrations, allows significant vertical deflection under “g” loads. This results in significant displacement of the propeller undermining maintenance of alignment between spinner and cowling, thereby requiring a larger operating clearance, and “bottoming out” of the mounts, i.e. the mounts come to the end of their allowable “soft” travel and become a more rigid connection defeating the purpose of their softness in shear.
For better performance the end mounts are sometimes focalized, i.e. disposed about the engine thrust axis with their individual axes orientated towards a (common) point near, or ahead of, the engine center of mass. In this manner the mounts are placed primarily in tension, or compression, rather than in shear, under normal aircraft “g” loading, and so are unlikely to “bottom out”; the axial stiffness remains high in order to absorb thrust load without excessive deflection; and the torsional stiffness, i.e. stiffness to oscillatory torque reaction loads, is low and substantially independent of “g” load and side load giving good isolation under all flight conditions. FIGS. 13A-13B show a known application of a DYNAFOCAL (trademark of the Lord Corporation) focalized system as applied to a horizontally opposed aircraft piston engine. FIG. 3 shows such a focalized system applied to another engine configuration, namely an “inverted” piston engine.
With attention to such focalizing, i.e. the position of the focal point (usually a little forward of the center of gravity), deflection of the propeller under normal “g” loading can be minimized or even practically eliminated in some cases. This is an advantage when designing spinner-to-cowling clearances. On occasion, such focalized mountings are employed with common support frame such as a prefabricated lattices of tubular struts and ties known as a “ring-beam”. While this predefines the mount configuration, it represents a complex and expensive additional element.